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CGS 3763 Operating Systems Concepts

Spring 2013

Dan C. MarinescuOffice: HC 30!

Office "ours: M#$% 11:30 # 12:30 &M

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'ast time:

(o%ay: C)* sc"e%u+ing

,e-t time

'ecture / Mon%ay anuary 2 2013

12116 2

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Sc"e%u+ing

4asic concepts5 sc"e%u+ing oecti8es.

Sc"e%u+ing po+icies 9irst#Come 9irst#Ser8e 9C9S;

S"ortest o 9irst S9;

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Sc"e%u+ing po+icies

4urst time

time re=uire% y a t"rea% to use t"e processorcore (ime s+ice=uantumtime a t"rea% is a++o>e% to use t"e processorcore

)reempti8e sc"e%u+ing& t"rea% cou+% e force% to re+ease t"e

contro+ of t"e processor

)o+icies

9irst#Come 9irst#Ser8e 9C9S S"ortest o 9irstS9

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9irst#Come 9irst#Ser8e% 9C9S;

("rea% 4urst (imeP1 2!

P2 3

P3 3 )rocesses arri8e in t"e or%er: P1P2P3

Gantt C"art for t"e sc"e%u+e:

$aiting time for P1 05 P2 2!5 P3 27 &8erage >aiting time: 0 F 2! F 27;3 17 Convoy effects"ort process e"in% +ong process

)1 )2 )3

2! 27 300

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("e effect of t"e re+ease time on 9C9S sc"e%u+ing

,o> t"rea%s arri8e in t"e or%er: P2P3P1

Gantt c"art:

$aiting time for P1 =6;P2 0; P3 = 3

&8erage >aiting time: 6 F 0 F 3;3 3 Muc" etterEE

)1)3)2

63 300

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S"ortest#o#9irst S9;

*se t"e +engt" of t"e ne-t urst to sc"e%u+e t"e t"rea%process >it"

t"e s"ortest time.

S9 is optima+minimum a8erage >aiting time for a gi8en set of

t"rea%sprocesses

(>o sc"emes: ,on#preempti8et"e t"rea%process cannot e preempte% unti+

comp+etes its urst

)reempti8eif a ne> t"rea%process arri8es >it" urst +engt" +ess t"an

remaining time of current e-ecuting process preempt. ?no>n as

S"ortest#

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-amp+e of non#preempti8e S9

("rea% aiting time 0 F 6 F 3 F 7;! !

)1 )3 )2

73 160

)!

12

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-amp+e of S"ortest#

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it" respect to conte-t s>itc"

ot"er>ise o8er"ea% is too "ig"

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it" time s+ice = 20

("rea% 4urst (ime

P1 3

P2 17

P3 6

P4

2!

(ypica++y "ig"er a8erage turnaroun% t"an S9 ut etter response

)1 )2 )3 )! )1 )3 )! )1 )3 )3

0 20 37 7 77 7 117 121 13! 1! 162

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(ime s+ice =uantum; an% conte-t s>itc" time

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(urnaroun% time function of time =uantum

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o

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Sc"e%u+ingpo+icy

&8erage >aiting timeti++ t"e o starte%

&8erage time insystem

9C9S 73 173

S9 !3 1!3

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)riority sc"e%u+ing

ac" t"rea%process "as a priority an% t"e one >it" t"e "ig"est

priority sma++est integer "ig"est priority; is sc"e%u+e% ne-t. )reempti8e

,on#preempti8e

S9 is a priority sc"e%u+ing >"ere priority is t"e pre%icte% ne-t C)*

urst time

)ro+emStar8ation / +o> priority t"rea%sprocesses may ne8er

e-ecute

So+ution to star8ation&ging / as time progresses increase t"e

priority of t"e t"rea%process

)riority my e compute% %ynamica++y

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